Sporadic errors when calling DLLs

[Helge]

I have a vb.net app that calls an "old" DLL that does some math (map
coordinate transformations), declared with an old fasioned declare statement
in my code.

Sometimes, but very rarely, the call fails with a COMException.
On the last dataset it failed 33 out of a total 4011085709 (4 billion)
calls.
My code catches the exception and tries one more time. Then it usually
works.

There is no information in the exception object that can tell me more about
what's wrong, as far as I can see.
When the exception occurs, and I run the code again with exactly the same
data input, it may go just fine, or maybe not. It's impossible to know.

Any help / hints would be appreciated!
Helge